1. Field of the Invention
The present invention relates to an apparatus for and method of manufacturing a semiconductor device, and a cleaning method for use in the apparatus for manufacturing a semiconductor device. Particularly, the invention relates to an apparatus for and method of manufacturing a semiconductor device, which are designed to perform hot processes, such as thermal oxidation, annealing, CVD and RTP, in manufacturing the semiconductor device, and also to a cleaning method for use in the apparatus for manufacturing a semiconductor device.
2. Description of the Related Art
In processes of manufacturing a semiconductor device, the steps of forming thin films on the semiconductor substrate (wafer) are very important. Each film-forming step utilizes thermal reaction, chemical reaction or the like between a feed gas and silicon, i.e., the representative material of the wafer, and a feed gas, or between various feed gases. So-called “hot processes,” such as thermal oxidation, thermal nitriding, annealing, rapid thermal process (RTP), and chemical vapor deposition (CVD), are particularly important.
Generally, these steps are carried out by introducing feed gases into the reaction furnace of a film-forming apparatus, in which one or more silicon wafers, i.e., semiconductor substrates, have been placed. To form films of desired properties (e.g., thickness, composition, resistance, etc.), the flow rates of the feed gases, the pressure and temperature in the reaction furnace and the processing time are preset. A controller controls the film-forming apparatus, causing the apparatus to operate in accordance with the preset values. In recent years, the internal microstructure of semiconductor devices has grown remarkably complex and acquired high component concentration. It is therefore very important to form high-quality thin films so that the semiconductor device that is a complicated and high-performance device may operate reliably in stable conditions. To this end, it increasing necessary to control, with very high precision, the various parameters (film-forming parameters) including the flow rates of feed gases, the pressure and temperature in the reaction furnace and the process time, all mentioned above.
As has been pointed out, it has become more necessary to control, with high accuracy, the film-forming parameters applied in the film-forming step in order to provide high-quality thin films. With ordinary film-forming apparatuses, however, some of the film-forming parameters cannot be controlled with so high a precision as desired, even if the controller for controlling the film-forming parameters is improved in terms of control ability.
A thermal oxidation process may be repeated several times (in several runs). In this case, the film-forming conditions are set so that a film may be formed each time (in each run) at the same conditions, such as oxidation temperature, flow rate of oxygen and pressure of oxygen. Theoretically, any thin film formed at one time should have almost the same thickness as the thin film formed at any other time. In practice, however, a difference in thickness, which cannot be neglected or allowed, may exist between the thin film formed in one run and the thin film formed in any other run.
Some reasons for this difference in thickness can be considered. For example, the partial pressure that the oxidizer assumes in the oxidization furnace may varies from run to run, due to any factor other than the flow rate of the oxygen being introduced into the oxidation furnace and the pressure of the oxygen introduced in the oxidation furnace. More specifically, if the process using water is performed in one run, some of the water may remain adsorbed in the furnace, not purged from the reaction furnace before the next run. In this case, the water acts as an oxidizer in the furnace. The oxide film formed while the water remains in the furnace is inevitably thicker than the film formed in a film-forming step at which water scarcely exists in the furnace.
In any film-forming apparatus that has a reaction furnace the interior of which is exposed to the atmosphere, the water in the atmosphere is taken into the reaction furnace when a wafer is brought into the furnace for each run. If so, the temperature in the furnace may differ from run to run, because the water concentration (humidity) in the atmosphere is not always the same at the start and end of any run.
The amount of the water adsorbed in the reaction furnace or of the water taken from the atmosphere into the furnace is extremely unstable. That is, it changes very much. Therefore, the amount of the water adsorbed or taken into the furnace is not set as a controllable parameter in the ordinary film-forming apparatuses. Even if the amount of the water is set as a film-forming parameter, oxide films may greatly differ in thickness so long as the apparatus that forms them performs a film-forming process using water or has a reaction furnace whose interior is exposed to the atmosphere.
A method many be devised, in which any very unstable factor, such as the amount of water outside the furnace, is not used as a film-forming parameter and a factor such as the components of the exhaust gas discharged from the furnace and containing feed gas used in the film-forming step is analyzed (measured, observed and monitored). Thus, the state of gas and the atmosphere, both in the furnace, during the film-forming step may be determined and then controlled to be appropriate ones. In this method, however, neither the state of gas nor the atmosphere in the furnace is accurately monitored.
This is because the component, concentration and the like of the feed gas introduced into the reaction furnace may largely differ from those the feed gas assumes outside the reaction furnace. That is, the components, concentration and the like of the feed gas may have different values each, before, during and after the film-forming step, depending on the thermal or chemical reaction that takes place during the film-forming step. Particularly, the more reactive or decomposable the feed gas is, the more greatly its components, concentration, etc. vary with time. Further, the composition, concentration and the like of the feed gas, thus analyzed, may greatly differ, depending upon the positions of the analyzers employed to analyze them.
The thickness of the film differs, from run to run, probably because of the residual feed gas accumulated in the reaction furnace. For example, the components of the feed gas fail to be reacted completely in one run and may adhere to the inner surface of the reaction furnace and may be solidify. When the next run is performed in this condition, any solid component of the gas, on the inner surface of the furnace, changes to gas due to the heat in the reaction furnace. In the next run, this gas mixes with the feed gas newly supplied into the reaction furnace. Consequently, the amount of feed gas in the reaction chamber increases over the constant value for each run. In other words, the amount of feed gas differs, from run to run. It follows that the thickness of the film varies, from run to run. The more runs are carried out, the more residue of the feed gas will likely be accumulated in the reaction furnace. This phenomenon is prominent in proportion to the number of runs carried out.
One film-forming apparatus may perform different film-forming steps. In this case, the material used to form a film differs from step to step. If the components of the material used in one film-forming step remain not completely reacted in the reaction furnace, it may be mixed with the feed gas in the next film-forming step, though it is unnecessary in the next step. If this component is mixed, the thin film formed in the next step may have not only a thickness greatly differing from the design value, but also properties totally undesired or extremely poor.